Electro-optical devices may be used to control an input optical signal to achieve a desired output signal. For example, Mach-Zehnder intensity modulators are presently being utilized in the telecommunications industry. For electro-optical intensity modulators, an electric field influences the index of refraction of material that defines a waveguide, so that the intensity of an optical signal at the output of the waveguide is modulated in correspondence with changes in the electrical field.
Polymeric materials have the potential of exceeding the requirements for advanced intensity modulators, optical switches, and other nonlinear optical devices. The beneficial characteristics offered by polymeric materials include large optical non-linearities and low dielectric constants. The large optical non-linearities permit use of low operating voltages (V.sub..pi.). Reductions in the dielectric constant permit simplification of high-speed modulation structures. Moreover, use of polymeric materials facilitates integration with electrical driving circuits and other optical devices, including semiconductor light sources and detectors.
Mach-Zehnder intensity modulators may be fabricated by forming three polymeric layers. Outside layers are cladding layers, while the center layer is used to form a waveguiding circuit. Photobleaching selected portions of the center layer of the substrate defines the waveguiding circuit. For example, the selected portions of the center layer may be exposed to light of the appropriate wavelength. The index of refraction is changed by the exposure. In this manner, an optical confinement is achieved along the waveguiding circuit. U.S. Pat. Nos. 5,119,450 to Ranganath et al. and 4,843,350 to Nazarathy et al., each of which is assigned to the assignee of the present invention, describe waveguiding circuits in which an input section is divided into a pair of parallel arms which converge to form an output section.
The photobleaching defines the optical path through the substrate, but the electro-optic effect is not achieved until the polymeric material is poled. In the poling process, nonlinear optical-active molecules are incorporated in the polymer matrix as guest/host, side-chain or cross-linked systems. The film is then poled by applying a high electric field across the substrate while it is heated to the glass-transition temperature of the material. The substrate is cooled to room temperature while the electrical field is maintained, thereby locking the molecules in the desired orientation.
After the poling process, the index of refraction along the waveguiding circuit can be varied in response to an applied electric field. Since the index of refraction of a material relates to the phase velocity of light in the material, an optical signal can be controlled by an electrical signal that defines the applied electrical field. Important goals in the design of an optical intensity modulator or switch include (1) reducing the modulation voltage required to achieve a desired electro-optic effect, (2) providing an impedance match between the source of the modulation signal and the electro-optical device, and (3) increasing the bandwidth of the device. As previously noted, the Ranganath et al. and the Nazarathy et al. patents describe waveguiding circuits that include intermediate portions having two arms. By poling the two arms to have opposite poling polarities, a push-pull effect can be obtained, allowing the switching voltage to be reduced by one-half. A single modulating signal is applied to the substrate to couple an index-affecting electrical field to both arms of the waveguiding circuit, but the reverse poling of the two arms induces opposite effects in the two arms with respect to indices of refraction.
Polymer Mach-Zehnder modulators with high bandwidth and low voltages have been achieved by using a microstrip transmission line to modulate the optical signal. Modulation bandwidth of 40 GHz has been reported, but not with the push-pull modulation approach that allows the fifty percent reduction in voltage. A lower power requirement is desirable, since it allows the equipment to use less costly drive circuitry. However, one difficulty in poling the two arms in opposite directions is that there is a tradeoff between fabrication concerns and operation concerns.
One fabrication concern involves dielectric breakdown during the poling process. Poling electrodes are positioned to induce separate and vectorially opposite electrical fields during the poling process. The two arms can easily be poled to have separate polarities when the two arms are spaced apart by a large distance. Referring to FIG. 1, a first poling electrode 10 and a second poling electrode 12 are shown on a side of a polymeric substrate 14. On the opposite side of the substrate is a ground electrode 16. DC voltages of opposite polarities are connected to the first and second electrodes, causing a first waveguide arm 18 to be poled in an opposite direction of a second waveguide arm 20 when the substrate is sufficiently increased in temperature. However, while the polymer of the substrate can withstand electrical fields of up to 2 MV/cm without breakdown occurring between the ground electrode 16 and one of the "hot" poling electrodes 10 and 12, breakdown between the two hot poling electrodes can easily occur if the separation between the two hot electrodes is on the order of the thickness of the substrate. This is because the potential between the two hot poling electrodes is approximately twice the potential between the ground electrode and one of the hot electrodes. In addition, the breakdown voltage of air is substantially less than that of the polymer. Thus, during the poling process, it is advantageous to have a large space between the first and second poling electrodes 10 and 12.
The poling electrodes 10 and 12 are then removed and may be replaced with a single transmission line having a microstrip geometry for applying a generally uniform electrical field across one or both of waveguide arms 18 and 20. In FIGS. 2 and 3, the single transmission line 22 is shown as extending over the two waveguide arms. Since the two arms are poled in opposite directions, a uniformly varying electric field generated by the transmission line 22 will result in push-pull modulation. The operation concern involves the electrical impedance. Conventionally, the thickness of a three-layer polymeric substrate 14 is between 8 and 16 .mu.m. If the thickness is 10 .mu.m and the width of the transmission line 22 is 50 .mu.m, the characteristic impedance of the structure is 25 ohms, assuming that the microwave index of the polymer is approximately 4. The impedance of the transmission line is inversely related to the width of the transmission line.
Most high frequency components, cables and instruments have default impedances of 50 ohms. Therefore, the 25 ohm transmission line 22 of FIGS. 2 and 3 will be impedance mismatched to a source of a microwave modulation signal. Impedance mismatches cause power loss and reflections that adversely affect performance. The conventional solution is to form a narrower transmission line 23 over only one of the waveguide arms 20, as shown in FIG. 4, but this arrangement is less electro-optically efficient than the one of FIGS. 2 and 3.
Briefly, closely spaced waveguide arms 18 and 20 are desirable during operation, since the uniform electrical field can be applied to the two arms by a narrow transmission line 22. Because linewidth and electrical impedance are inversely related, impedance matching with the signal source is more easily achieved if the linewidth is narrow. However, fabrication concerns dictate widely spaced waveguide arms, since dielectric breakdown is less likely to occur during a process of poling the two arms in opposite directions with large fields. Optical modulators having a 40 GHz bandwidth have been demonstrated using a thin 50 ohm microstrip transmission line to couple an electric field with a single arm, but the absence of the push-pull effect necessitates higher modulating voltages. One possible solution is to pole the two arms of a Mach-Zehnder device by applying the poling voltages in alternating pulses, thereby reducing the susceptibility of the poling process to dielectric breakdown. However, this technique is difficult to implement. Moreover, an operation consideration that has not been previously identified is optical crosstalk. Light energy from one waveguide arm may escape and enter the other waveguide arm. Thus, there is a second factor that leads away from closely spacing the waveguide arms.
What is needed is a low voltage, high bandwidth electro-optical device that decreases susceptibility to dielectric breakdown during the poling process and that reduces optical crosstalk during operation, while permitting flexibility in the structure of a transmission line with regard to achieving a desired electrical impedance of the transmission line.